Heavy endurance exercise rapidly leads to exhaustion. Many factors, some of which remain ill-defined, influence the ability to tolerate such exercise. Previous evidence suggests that one of these factors may be related to the amount of work done by the ventilatory muscles. This project will first define the role that normal levels of ventilatory muscle work may play in endurance exercise tolerance. This will be done by comparing exercise performance in normal conditions with that measured when the muscles of breathing are "unloaded" with low density gas. Next, the mechanism that links the level of breathing work to exercise tolerance will be examined. Possible mechanisms include lactate production by respiratory muscles, the oxygen cost of breathing, and ventilatory muscle fatigue. Each of these possibilities will be tested. Previous work points to ventilatory muscle fatigue as the most likely link between the level of breathing work and exercise tolerance. This hypothesis will be tested by analysis of the EMG for evidence of fatigue from both inspiratory and expiratory muscles, while ventilation is sustained voluntarily at levels matched to those seen in exercise. If ventilatory muscle fatigue is indeed sizable in heavy endurance exercise, and if this fatigue influences exercise tolerance, several testable predictions can be made. One is that the ability to sustain high levels of ventilation should be greater in endurance athletes than in untrained persons. Thus standard endurance exercise training, when applied to sedentary individuals, should increase their ventilatory muscle endurance. Finally, specific training of the ventilatory muscles alone should improve performance of standard forms of heavy endurance exercise.